The present invention relates to an improvement in an amplifier circuit provided with a single ended push pull (hereinafter referred to as an SEPP) circuit.
SEPP circuits are push pull circuits operative in parallel for a load and in series for a power supply. As an example of conventional SEPP circuits, there is known a circuit in which fixed resistors are connected to emitters of respective output transistors in order to increase d.c. stabilities of the output transistors, respectively, and bias generators including a diode and a semi-fixed resistor connected in series are connected to the bases of the respective output transistors in order to give a d.c. bias to the bases of the output transistors (e.g., Japanese Patent Application Laid Open Publication No. 42540/78 Official Gazette).
However, the above mentioned conventional SEPP circuit has some drawbacks that the stability of the bias is insufficient, that an output loss is produced because resistance value of each emitter resistor cannot be small, and that there is produced a distortion larger than an allowed distortion when used as the output stage, and the like.
As an SEPP circuit to eliminate these drawbacks, there is known a circuit in which differential amplifiers are inserted between the signal input terminal and the bases of respective output transistors (e.g., see Japanese Utility Model Application Laid Open Publication No. 69121/81 Official Gazette or Japanese Utility Model Publication No. 41622/84 Official Gazette).
While the above mentioned second prior art can overcome the drawbacks with the first prior art, it a the drawback that because the power supply for delivering a power to the output transistors is adapted to double as the power supply for the differential amplifiers, it is required for the differential amplifier to have a high withstand voltage.
On the other hand, when attention is drawn to the case where a load for a low impedance (an impedance lower than a load impedance which can be driven by a single power amplifier) is driven by a plurality of power amplifiers connected in parallel using the above mentioned SEPP circuit, in the case where a negative feedback (NFB) is applied to each power amplifier, load cannot be directly connected to the output terminal thereof. In view of this, mixing resistors (or impedances) are generally connected between respective output terminals.
However, the presence of the mixing resistors leads to the problem that any output loss (so called mixing loss) in the mixing resistors may be produced. Particularly, in the case of an amplifier having a high amplification factor, any difference between outputs from respective amplifiers in the parallel driving or running is apt to occur. To prevent this, the mixing resistor is required to have a large mixing resistance value. However, an increase in the mixing resistance value would lead to a further increase of the mixing loss.